Human Physiology, 14th edition (2016)

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19.1 NUTRITIONAL

REQUIREMENTS

The body’s energy requirements must be met by the caloric

value of food to prevent catabolism of the body’s own fat,

carbohydrates, and protein. Vitamins and minerals do not

directly provide energy but instead are required for diverse

enzymatic reactions.

required to raise the temperature of one cubic centimeter of

water from 14.5 8 to 15.5 8 C. The amount of energy released

as heat when a quantity of food is combusted in vitro is equal

to the amount of energy released within cells through the pro-

cess of aerobic respiration (chapter 4; see fig. 4.13). This is

4 kilocalories per gram for carbohydrates or proteins and

9 kilocalories per gram for fat. When this energy is released by

cell respiration, some is transferred to the high-energy bonds

of ATP and some is lost as heat.

Metabolic Rate

and Caloric Requirements

The total rate of body metabolism, or the metabolic rate, can

be measured by either the amount of heat generated by the

body or the amount of oxygen consumed by the body per min-

ute. This rate is influenced by a variety of factors, discussed in

section 19.2 (see fig. 19.5 ). For example, the metabolic rate is

increased by physical activity and by eating. The increased rate

of metabolism that accompanies the assimilation of food can

last more than six hours after a meal.

Body temperature is also an important factor in determin-

ing metabolic rate. The reasons for this are twofold: (1) tem-

perature itself influences the rate of chemical reactions, and

(2) the hypothalamus contains temperature control centers,

as well as temperature-sensitive cells that act as sensors for

changes in body temperature. In response to deviations from a

“set point” for body temperature (chapter 1), the control areas

of the hypothalamus can direct physiological responses that help

correct the deviations and maintain a constant body temperature.

Changes in body temperature are thus accompanied by physi-

ological responses that influence the total metabolic rate.

Marty, sixty-five years old, asked his physician what his

psoriasis cream contained and was told that it was a

vitamin D derivative. Marty responded that he also took

vitamin D capsules to supplement his diet. “Good,” his

physician responded. “It may help your psoriasis, and it

may also help you to prevent bone loss.” The physician’s

tone became more serious when he added, “But I’ve told

you before that you need to lose weight! You have a BMI

of 34 and a fasting plasma glucose of 120 milligrams per

deciliter, putting you in danger of diabetes as well as car-

diovascular and other diseases. If that happens, you’ll

have to take other medications for the rest of your life.”

The new terms and concepts you will encounter

include:

• Vitamin D actions, osteoporosis and osteomalacia


• BMI, metabolic syndrome, diabetes mellitus, and
  insulin resistance

Clinical Investigation

CLINICAL APPLICATION

Hypothermia occurs when the body temperature drops below

35 8 C (95 8 F). This induces compensatory shivering and sym-

pathetic nerve responses (raising pulse and blood pressure)

at first, but shivering stops when hypothermia becomes more

severe. Then the person becomes drowsy and weak, even-

tually losing consciousness and dying if countermeasures

are not taken. However, in the technique of hypothermic

circulatory arrest, the body temperature is intentionally

dropped, generally to 15 8 to 20 8. This is medically induced for

heart and aorta surgery because it greatly reduces the meta-

bolic rate of the central nervous system, allowing the circula-

tion to be stopped during surgery.

LEARNING OUTCOMES

After studying this section, you should be able to:

1. Describe how various conditions affect the
   metabolic rate.


2. Describe the caloric and anabolic requirements of
   the diet and the functions of specific vitamins.


3. Identify the nature and significance of free radicals.

Living tissue is maintained by the constant expenditure of

energy. This energy is obtained directly from ATP and indirectly

from the cellular respiration of glucose, fatty acids, ketone bod-

ies, amino acids, and other organic molecules. These molecules

are ultimately obtained from food, but they can also be obtained

from the glycogen, fat, and protein stored in the body.

The energy value of food is commonly measured in

kilocalories, which are also called “big calories” and spelled

with a capital letter (Calories). One kilocalorie (kcal) is equal

to 1,000 calories; one calorie is defined as the amount of heat

The metabolic rate (measured by the rate of oxygen con-

sumption) of an awake, relaxed person 12 to 14 hours after

eating and at a comfortable temperature is known as the basal

metabolic rate (BMR). The BMR is determined primarily by a

person’s age, sex, and body surface area, but it is also strongly

influenced by the level of thyroid hormone secretion. A person

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